Always-2-spaces is the only rule that will always work. The nuance around 2 spaces vs. prior-arg-alignment is unsustainable in the face local rebindings of core fn/macro names and people's special expectations around how their own macros should be treated.
Trying to do anything else results in things like this:
where indentation is modified based on var naming conventions (here, anything prefixed by 'with-' is indented as per `with-open`, instead of whatever the clojure-mode default is), and things like this:
where one ends up configuring one's personal editor to indent particular forms differently. Of course, those changes are only local (so won't be applied when someone else touches the code, or even when you touch the code using a different editor or on a different computer), must be constantly maintained in order to stay abreast of changes in the projects that one works on, and are applied globally (so you can't have a macro `foo` in project A indented differently than a function `foo` in project B).
Always-2-spaces is the only rule that will always work. :-)
Cheers,
- Chas